This is one symptom of generally weird behaviour of glissandi on pretty much every score I write. They get displayed and then all of a sudden I don't see them any more when I get to the relevant passage later. I have no idea whatsoever as for what might be the cause for this.

My observation is that glissandi need a certain horizontal space between the notes to be displayed properly.
If this space is reduced during formatting or copying, for whatever reason, the glissando is not displayed.
Manually increasing the space between the notes re-displays the glissando at some point.
Glissando display should have a way of enforcing this minimal required horizontal spacing.

OS: Windows 10 Version 2009 or later, Arch.: x86_64, MuseScore version (64-bit): 4.2.1-240230937, revision: github-musescore-musescore-d757433

Glissando lines do have an enforced minimum spacing.

It would be useful to have a specific score where this problem occurs, for testing. Again, I can't reproduce this from scrach.

Here you are, as it opens in measure 8 there is a glissando. Only the line shows, move the C to the right to see "gliss." showing.
Now copy measure 8 to, for example, measure 12. All indications of glissando in measure 8 disappear. In measure 12 there is also no indication. Move the C to the right in both places to get the glissando to appear.
What it does in the corresponding part is pretty random.
I hope this helps.
Don
GlissTest.zip

OS: Windows 10 Version 2009 or later, Arch.: x86_64, MuseScore version (64-bit): 4.2.1-240230937, revision: github-musescore-musescore-d757433

This sounds similar to a problem I've experienced. If you open the following score
gliss-bug.mscz.zip
(rename from .zip to .mscz), and try to add a dynamic in the first measure, the gliss. lines disappear. Undo does NOT restore them. Moving notes around can also cause them to disappear and reappear randomly.
But then it didn't happen when creating a brand new score with the same notes!

Confirmed the above problem (adding a dynamic to the attached score and having the glissandi lines disappear) still happens with master. And other users have definitely experienced similar issues with disappearing lines.